Semiconductor devices are formed on silicon substrates and are typically multi-layered, having numerous metalization layers separated by numerous insulating oxides and interconnected with vias or contact holes. For instance, an interconnect for a typical multi-layer device is formed by depositing and patterning a first metal layer over the device, depositing an intermediate oxide over the patterned first metal layer, photolithographically defining a contact hole in the oxide, and depositing a second metal layer over the oxide that fills the contact hole and contacts the patterned first metal layer.
Often undesirable steps or undulations must be removed from the silicon substrate or from one of the metal or oxide layers before another layer can be formed thereon. To remove steps or undulations, the silicon oxide or metal is preferably planarized, removing any steps or undulations formed therein, prior to deposition of a layer thereon. Planarization is typically performed mechanically by forcing the semiconductor wafer face down against a polishing pad which is saturated with a polishing slurry, and by moving the polishing pad relative to the wafer. The relative movement between the polishing pad and the wafer, combined with the polishing slurry chemically and mechanically removes layers of material and is continued until the steps or undulations are removed. This process is generally referred to as chemical mechanical polishing (CMP).
Suitable performance of CMP generally requires that an appropriate amount of polishing slurry be delivered to the polishing pad. For example, film removal rates are strongly dependent on the amount of slurry dispensed. However, slurry dispensing rates often vary due to fluctuations in facility slurry supply pressures (e.g., at the tool inlet). To aid in metering the flow of polishing slurry so that an appropriate amount is delivered to the polishing pad, it has been proposed to detect the flow rate of the polishing slurry in a slurry supply line. However, previously proposed techniques for detecting the rate of flow of polishing slurry may lead to clogging of the slurry supply line and/or may create shear in the flow of polishing slurry, thereby resulting in particle agglomeration within the slurry. This, in turn, may lead to micro-scratching of the wafer during CMP, resulting in lower device yields.